1. Field of the Invention
The present invention relates to implantable electrochemical sensors and, more particularly to an implantable sensor that utilizes a biocompatible electroconductive case as an auxiliary electrode in a three electrode system.
2. Description of Related Art
The chemical reaction most commonly used in enzyme coupled glucose sensors is the glucose oxidase mediated catalytic oxidation of glucose by atmospheric oxygen to produce gluconolactone and hydrogen peroxide (equation 1): EQU C.sub.6 H.sub.12 O.sub.6 +O.sub.2 +H.sub.2 O.fwdarw.C.sub.6 H.sub.12 O.sub.7 +H.sub.2 O.sub.2 ( 1)
A typical two electrode electrochemical cell comprises an anode, otherwise known as the measuring electrode, and a cathode, otherwise known as the reference electrode. In the presence of excess oxygen, the quantity of hydrogen peroxide produced in the reaction of Equation 1 will be a direct measure of the glucose concentration. The hydrogen peroxide is detected as it is reoxidized at the measuring electrode which is maintained at a sufficient positive potential to carry out the following reaction (equation 2): EQU H.sub.2 O.sub.2 -2e.sup.- .fwdarw.O.sub.2 +2H.sup.+ ( 2)
Glucose detection is dependent upon the measurement of electrons removed from hydrogen peroxide in equation (2). The electrode is normally formed from a noble metal such as gold or platinum.
The complementary electrochemical reaction, a reduction, takes place at the cathode or reference electrode, usually a silver/silver chloride electrode, as shown in Equation 3. EQU 2AgCl.sub.(s) +2e.sup.- .fwdarw.2Ag+2Cl.sup.- ( 3)
For the two electrode electrochemical cell, the magnitude of the current flow for the oxidation of hydrogen peroxide at the measuring electrode must be equal to and balanced by an equivalent flow of current of the opposite sign at the reference electrode. However, in the two electrode electrochemical cell the conventional Ag/AgCl reference electrode's potential can become unstable during operation. The reference electrode potential is a function of the concentration of the silver chloride. If the amount of current flowing through the reference electrode is sufficiently large that the Ag metal or the AgCl solid is exhaustively consumed the potential for the reference electrode will not be constant. As a result, there is a need for a system that limits the amount of the current through the reference electrode such that the Ag metal or AgCl solid is not exhaustively consumed and instability in the reference electrode's potential does not result.
Prior art attempts at solving the problems associated with two electrode systems have employed a third electrode as an auxiliary electrode. U.S. Pat. No. 5,352,348 to Young et al. discloses a glucose sensor that employs a measuring electrode, a silver electrode and a silver/silver chloride electrode.